1. Field of the Invention
The invention relates to a device for tensioning a nut and screw bolt combination, particularly such a combination arranged on a pitch circle, wherein the tensioning device includes a support ring having recesses for receiving the nut together with the screw bolt, a separate turning unit fastened to the ring for rotating the nut, a lifting cylinder having a piston and arranged at the support ring and means responsive to the piston for engaging the screw bolt in a positively locking manner.
2. Description of the Prior Art
Such a tensioning device is used, for example, for closing or opening the head covers of reactor vessels, and is disclosed in the German Offenlegungsschrift No. 24 28 847. In this known device, the screw bolt is tensioned by lifting cylinders located at the support ring. When pressure is admitted to these cylinders they stretch the screw bolt via pressure bridges and coupler jaws which are inserted into corresponding engagement surfaces at the screw bolt in a form-locking manner. In the tensioned condition the nut of the screw bolt is turned all the way to its end position, so as to retain the tension placed on the screw bolt. During tensioning, however, the form-locking engagement of the coupler jaws with the screw bolt necessitates an external force which must be maintained as a counter force during the entire tensioning process. Moreover, this force must be controlled so that no unilateral load is set up which could result in uncontrolled distortion of and damage to the screw bolt. The generation of such controlled counterforce thus requires a highly complicated structure.
A further type of automatic screw tensioning device is also disclosed in the German Offenlegungsschrift No. 22 58 859. In this type of device, the pistons of the lifting cylinders act on the screw bolt via a tensioning nut and a separate turning unit serving as the drive is provided for the tensioning nut as well as for the nut of the screw bolt. While this arrangement thus permits the screw bolt to be loosened, tensioned or released fully automatically, the dual drive required for each screw bolt is very expensive making the overall device costly. It is, therefore, an object of the present invention to provide a tensioning device of the above type which does not necessitate the use of a separate drive for the tensioning nut nor the use of a counterforce.